HiRISE images pass through several layers of calibration. The purpose of calibration is to make the image appear more realistic, more like how the surface really is. It makes science more possible, in general terms (Not limited to HiRISE) to determine the composition of surface materials, easier discoveries of surface features, and as a whole makes the images more useful. Irregularities arise from the camera system, from the optics, from any number of things. These features are mostly corrected out if calibration is done correctly. There has been some question as to what an image looks like before calibration, and after, and I’d like to give you an example, using our picture from Victoria Crater.

This is an example of a HiRISE image pre-calibration. Notice the vertical streaks across the image. Harder to see is something known as dark current, which appears with any camera if a black picture is taken with it. There is also variations of pixel sensitivity, all of which must be accounted for. This doesn’t include the special calibrations needed for color images, but there are some as well.

This is the same image in roughly the same spot, taken after the calibrations have been done. Note that there is no longer any vertical lines, at least, none that are obvious. That is one of the things that is corrected during calibration. There are several other things which are done as well, but they aren’t as visible, but the image as a whole has been made more scientifically useful. There is always the risk of cutting into the useful part of an image, but HiRISE has several advantages in this respect. HiRISE has a high SNR, or Signal to Noise Ratio. SNR is exactly what it sounds, the ratio of the signal (In our case the light reflected from the Martian suface) to the noise (Which can be anything from Thermal noise, Cosmic Background Radiation, etc) With HiRISE’s large mirror, and pushbroom CCD arrays, it is able to achieve what hasn’t been achievable in other cameras. What this means is that even relatively dark surfaces are still visible to a degree with HiRISE, which has not been the case with most previous imagers.

Calibration images are taken to assist in this process. They are mostly taken by pointing HiRISE at the dark side of Mars, with various temperatures, recording what image results. There are a variety of what’s known as Stim lamps to give an approximation of white light as well, which also assist with calibration. The results from these images (Which are all very small) are placed into a database to know the correct way to calibrate each image. All of these are done so that everyone can see Mars the way that it was meant to be seen.